This application claims the priority of PCT/EP98/01491, filed Mar. 14, 1998 and 197 12 356.2, filed Mar. 25, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for reducing harmful exhaust-gas emissions of a spark-ignition engine operated with a lean fuel/air mixture and having direction fuel injection. More particularly, the inventors relate to a method wherein under part load the engine being operated in the stratisfied-charge mode with a lambda value of .lambda.&gt;1, and, in order to reduce nitrogen oxides adsorbed on a catalyst, a transition to a homogeneous mode with air ratios of .lambda.&lt;1 being carried out at specific intervals by at least one of throttling of the air quantity supplied and/or by an increase in the fuel injection quantity.
It is known that NOx storage catalysts are suitable for the aftertreatment of exhaust gas in so-called lean-burn engines. With lambda values (.lambda.)&gt;1, nitrogen oxides are stored, lambda representing the ratio of the air quantity supplied to a theoretical air requirement for combustion. In order to reduce the nitrogen oxides adsorbed on the catalyst, a jump to air ratios .lambda.&lt;1 is necessary at specific intervals.
In conventional lean-burn engines which have a homogeneous fuel/air mixture and the leanness of which can be brought at least to lambda values of .lambda.=1.6, in principle enrichment can take place while the air quantity remains the same. In this case, the engine torque can be kept constant by retarding the ignition timing. By additional adjustment of the throttle valve for the air supply, the retard of the ignition timing can be limited to only a few work cycles.
A spark-ignition engine with direct injection can, however, be made substantially leaner. In order to obtain consumption values which are as low as possible, such an engine is run unthrottled, that is with a constant air quantity, in the usual way. This engine, therefore, is run in the so-called stratified-charge mode in most operating states. In contrast to homogeneous operation with a lambda value of .lambda.=1, with the fuel already being injected during the suction stroke of a piston, in charge stratification or a stratified-charge mode, the fuel is injected only just before ignition in the compression stroke of the piston. Due to largely unthrottled operation, the engine is operated with a correspondingly high air excess.
Alternation between charge stratification and homogeneous operation, and vice versa, is designated as operating-mode alternation. Since exhaust-gas aftertreatment with a three-way catalyst is not possible in the case of a lean exhaust-gas composition of this kind, an NOx storage catalyst, for example, is used instead, which, as mentioned, then has to be regenerated at regular intervals when there are air ratios .lambda.&lt;1, in order to desorb the adsorbed nitrogen oxides again and subsequently reduce them.
This means that, in the partial-load mode, a transition to air ratios of .lambda.&lt;1 and back again, with the air quantity remaining the same, would be accompanied by a very pronounced load change. For this reason, the intake air quantity must be adapted accordingly, and, at the same time, there must also be a transition from the operating mode "stratified-charge mode" to a "homogeneous mixture mode". Operation with a homogeneous mixture is possible only when there are lambda values of .lambda.&lt;1.6.
It is known from EP 0,752,521 A1 to adjust the throttle valve accordingly in order to adapt the air quantity. Here, however, there is the problem that, because of the air quantities stored in the flow path between the throttle valve and the intake region of a combustion space or as far as the combustion space, a correspondingly long time is required until air-quantity changes brought about by the adjustment of the throttle valves have an effect in the combustion space. The engine has to be run in an adverse way according to intermediate values for a relatively long period of time. The same applies to a subsequently repeated increase in the air quantity from rich to lean and from the homogeneous mode to the stratified-charge mode. Moreover, each cylinder has two intake ducts having throttle valves regulated in a load-dependent manner.
DE 27 47 885 A1 discloses a method for the reduction of harmful exhaust-gas emissions of a multi-cylinder internal combustion engine, in which one half of the cylinders is to be operated with a rich mixture and the other with a lean mixture. The exhaust-gas intervals with the same composition thereby become longer or their frequency becomes lower, depending on the number of cylinders in the internal combustion engine. The degree of conversion of a selective catalyst is thus to be markedly improved or the emission of harmful exhaust gases reduced. A similar method is described in GB-C-1,492,228.
DE 39 40 752 A1 describes a method for controlling a spark-ignition engine without a throttle valve with the aid of variable inlet-valve opening durations. This method is intended to adapt the masses of air and fuel to one another in a permanently optimum manner in all driving states, even when there are sudden changes in the accelerator-pedal signal, in order to achieve a high degree of driving comfort, without misfires, and low harmful gas emission. Regulation to the lambda value .lambda.=1 is carried out for this purpose. The inlet-valve opening durations are utilized for regulation due to the absence of throttle valves in the internal combustion engine described. By regulating to the lambda value .lambda.=1, combustion can take place under stoichiometrically highly favorable conditions, a process notable for its lower pollutant emission, in particular lower proportion of carbon monoxide.